Problem: If $f(x) = 2x^2 + 8$, what is the positive value of $x$ for which $f(x) = 136$?
Explanation: We seek a positive solution for the equation $2x^2+8=136$, or $2x^2=128$. Dividing both sides by 2, we see that $x^2=64$, and thus the positive value of $x$ is $\boxed{8}$.